1. Field of the Invention
The present invention relates to heat-resistant and anticorrosive steel materials such as plates, pipes, joints, clamps, bolts, nuts and etc. which are covered with a plurality of metal-plated layers and which excels in the uniformity of processability and anticorrosiveness.
2. Description of the Prior Art
Heretofore, it has been usual that steel materials used as plates, pipes, joints, clamps, bolts, nuts and etc. used for automobiles and other various kinds of mechanical apparatuses are often plated with Zn to form a Zn-plated surface and then a chromate film is formed to cover the Zn-plated surface.
However, since a higher degree of anticorrosiveness has come to be required of these steel materials, especially for automobiles, the formation of only a Zn-plated layer has been found insufficient with respect to anticorrosiveness and in order to improve the anticorrosiveness of these materials, alloy platings such as Sn/Zn, Zn/Ni and etc. or a combination of such metal-plated layers and the Zn-plated layer has come to be employed. Thus, in Japanese Laid-Open Patent Publication No.H2-120034 there is proposed a heat-resistant and anticorrosive multilayer metal-plated steel pipe having, on the outer surface thereof, a Ni-plated layer, a Zn/Ni alloy-plated layer and a chromate film in that order.
However, a single layer of Zn/Ni alloy plating has the problem of lacking heat-resistivity and anticorrosiveness and a single layer of Ni+Zn/Ni alloy has the problems that although it has a favorable degree of heat-resistivity and anticorrosiveness, when the steel material has a complicated three-dimensional configuration, an acid bath such as a chloride bath or sulfuric acid bath is used so that the resultant plated film lacks uniformity with the result that the thickness of the plated film at the end portions of the material becomes large which reduces the processability of the material, while the film thickness becomes small at concave portions which reduces anticorrosiveness. Further, the rate of eutectoid becomes high at the concave portions and so the formation of a chromate film representing coloring property or reactivity becomes worse thereby deteriorating the uniformity of the external appearance of the material as a whole. In addition, there is also a problem that where an alkaline bath is used, while the uniformity of the chromate film is favorable, the adhesion between Ni and Zn/Ni reduces upon bending so that in a high temperature environment such as in the engine compartment of an automobile, insufficient processability and heat-resistivity of the material have been displayed so far.